Fish found leaving the sea to evolve into land animals

Fish out of water? That's exactly where these fish want to be.

A blenny, the same type of fish that is learning to live on land.
(Photo: Wiki Commons)

It's one of the great questions of evolutionary science: how and why did fish eventually evolve to become land animals? Well, it turns out that we might be witness to a similar event happening right now, right before our eyes.

Researchers at the University of New South Wales in Australia have been studying several species of blenny fish, or "blennies," that live on Rarotonga, the largest of the Cook Islands, and have found them to be surprisingly adept at life on terra firma. Not only do the researchers believe that these fish are currently in the process of evolving into land animals, but by studying them we may finally glean evolutionary answers about why any fish ever crawled out of the sea, reports New Scientist.

Moving from sea to land is no small endeavor, and it can't be done in one giant leap. For blennies, it's no different. These fish are still at home in the water, spending plenty of time foraging around in pools during low tide. But when the tide rises, these adorable little swimmers wiggle onto coastal rocks to stay high and dry. They have gills, so still need water to breathe, but they're able to get all the oxygen they need from ocean spray, splashes, and small moist crevices.

But why do they prefer the land at high tide? Researchers suspect that it's because high tide is when the predators move in. Larger fish that hunt blennies include coral reef prowlers such as flounders, groupers, wrasses and moray eels. But none of those fish can follow the blennies ashore.

To test their theory, the researchers made plasticine models of blennies that looked just like the real thing, and submerged them in tide pools. Sure enough, when the tide rolled in, there was a virtual plasticine blenny massacre. The lures were later discovered replete with puncture wounds, bite marks and whole parts missing. So it would seem that the principal reason blennies are moving to the land is because they're being driven there to escape predators.

"Our study of blennies on Rarotonga is the first to examine the pressures driving fish out of the water," said lead researcher Terry Ord.

Of course, there are predators that live on land, too. But on an island like Rarotonga, land predators are far more scarce, which makes it the ideal environment for this sort of sea-to-land adaptation to take place. Ord and his team believe that when the first fish began crawling on land some 400 million years ago, a similar balance of evolutionary pressures were at play. The sea was teaming with scary predators, but there were fewer threats on land.

“If you never look over the fence, you’ll never know that the grass is greener,” explained Ord. “However, if you’re forced to the other side to escape something, you may then realize it has additional benefits and want to stay there and adapt.”

There were likely other pressures at play when our ancestors first used their fins to pull themselves ashore too. Maybe they were looking for more food, for instance. But this study offers us a unique opportunity to actually study one of these evolutionary pressures at work in real-time.